The present invention relates to a process for forming a film of a hydrolysed ethylene-vinyl acetate copolymer.
A film of a hydrolysed ethylene-vinyl acetate copolymer having an ethylene content of 25 to 55% by mole and a degree of hydrolysis in the vinyl acetate component of at least 90% by mole has an excellent transparency and oxygen impermeability, and is useful as wrapping or packaging material for foods, medical supplies, industrial chemical reagents and the like. It is often used as composite material, e.g. a laminate with plastic film, paper, metal foil or the like.
Many processes can be employed for forming a film of the hydrolysed ethylene-vinyl acetate copolymer. In these processes, coating process in which a solution of the copolymer dissolved in a solvent is applied to a substrate is considered to be more effective on the point of the advantages that a thinner film can be formed, that a film can be easily formed on even a substrate of complicated shape such as a hollow container and that applying operation can be carried out by means of simpler and cheaper devices.
As to the coating process, there have been proposed many types of solvent for the copolymer. For example, water-containing propyl alcohol and water-containing butyl alcohol described in Japanese Examined Patent Publication (Tokkyo Kokoku) No. 23911/1971 and No. 48489/1972 are considered as good solvents in view of transparency of a film.
However, in case of employing such water-containing alcohol as a solvent, the applying operation must be carried out with keeping the solution at a high temperature of 50.degree. C. to obtain a good transparent film. Also, the solution including such water-containing alcohol has defects that a storage stability is worse and the solution must be subjected to re-heating to dissolve the deposited products at a time of use.
Also, concentrations in the solution are apt to be changed due to the solvent evaporation. Therefore, the applying operation must be carried out in a closed system requiring a careful process control. Further, in an aspect of properties such as transparency and gas impermeability, a film prepared from the solution employing water-containing alcohols as a solvent does not always satisfy the requirements of quality which recently becomes strict more and more. Consequently, it is desired to produce a film of the hydrolysed ethylene-vinyl acetate copolymer which is further improved in the properties.
It is an object of the present invention to provide a process for forming a film of a hydrolysed ethylene-vinyl acetate copolymer having remarkably advanced transparency and gas impermeability, in which applying operation is carried out at a normal temperature without requiring any special device or complicated operation at a time of application.
These and other objects of the present invention will become apparent from the description hereinafter.